The present invention concerns structural units, and in particular concerns interconnected concrete structural units, although in its broader aspects the present invention is not limited to only concrete materials.
In the past, concrete pipe sections have been connected to form sewer lines, drains or culverts by using a male and female type of connection such as a bell and spigot or tongue-and-groove. The spigot or tongue is a tapered end while the bell or groove is flared. The spigot end on one pipe is inserted into the flared end on another pipe. Often, rubber gaskets have been placed on the spigot with the gasket rings being deformed and compressed in making the joint in order to seal the joint. Also, the joint can be filled with mortar or asphalt. One or both of the rubber gasket and the mortar/asphalt are used to seal the pipe joint. However, the pipe settles over time and the joints tend to open causing the mortar or asphalt to crack, and/or the gasket sealing force to be lost (decreased).
In order to provide a more rigid and lasting connection, sections of concrete pipe were cast with the ends of reinforcing rods projecting out of the pipe at one end and with an aperture or recess at the other end for receiving the end of the reinforcing rod from a section of pipe to be joined. The ends of the reinforcing rods were threaded and washers and threaded nuts were used to fasten the sections together. This method presents a serious problem in that the threaded ends of the reinforcing rods can be damaged in shipping or in joining sections of pipe together, destroying the usefulness of the entire section of pipe.
Other techniques have also been used, such as hook-and-eye methods; however, these also suffered like the exposed reinforcing rods from damage in shipping or in assembling pipe sections together.